Hofmeister Effects Of Ion Adsorption On Montmorillonite Surfaces

Ion adsorption is a physicochemical process that generally exists in soil.Regarding the thermodynamics and kinetics of ion adsorption in soil or clay minerals,a lot of research work has been carried out,and many different thermodynamic and kinetic models have been used to describe the process of ion adsorption in soil or clay mineral.Because the ion adsorption in soil is difficult to be in a true thermodynamic equilibrium state,the kinetic study of ion adsorption is particularly important.As we all know,a large amount of charge on the surface of soil or clay minerals can form a strong electric field in the space near the surface.Recent studies have found that the strong electric field may change the energy and quantum state of the electrons outside the nucleus of the adsorbed ion and induce the non-classical polarization of the adsorbed ion.Hofmeister effects are ubiquitous in physics,chemistry and biology,and play an important role in interfacial interactions.Some studies believe that non-classical polarization of ions is another source of Hofmeister effects.In this study,based on the kinetic model considering the non-classical polarization of ions,different montmorillonite samples were selected?Cu2+-montmorillonite and Zn2+-montmorillonite saturated samples?,through the comparative study of the adsorption reaction of monovalent ion Li⁺,Na⁺,K⁺,Cs⁺and divalent ion Ca²⁺,Mg²⁺on different montmorillonite surfaces,the essential reason of Hofmeister effects was explored by using the established ion adsorption model.And then the Hofmeister effects and mechanism of monovalent and divalent metal ions adsorption on montmorillonite surface were revealed.The results indicated:?1?The adsorption of Mg²⁺and Ca²⁺on the surface of Cu²⁺-montmorillonite and Zn2+-montmorillonite is a first-order kinetic process under the action of weak electrostatic force,and the average adsorption rate and equilibrium adsorption amount show a significant Hofmeister effects,showing the trend of Mg²⁺<Ca²⁺.For example,when the electrolyte concentration is 0.01 mol L-1,the average adsorption rates of Mg2+and Ca2+on the surface of Cu2+-montmorillonite are 10.05 and 12.03 mmol kg-1 min-1,respectively,for equilibrium adsorption amounts are 468.98 and 497.49 mmol kg-1.The average adsorption rates of Mg²⁺and Ca²⁺on the surface of Zn²⁺-montmorillonite are10.38 and 12.50 mmol kg-1 min-1,and the equilibrium adsorption amounts are 430.13and 477.88 mmol kg^-1,respectively.?2?The adsorption of Li⁺,Na⁺,K⁺,and Cs⁺on the surfaces of Cu²⁺-montmorillonite and Zn²⁺-montmorillonite are all first-order kinetic processes under the action of weak electrostatic force,and the average adsorption rate and equilibrium adsorption amount show a significant Hofmeister effects,showing the trend of Li+<Na⁺<K⁺<Cs⁺.When the electrolyte concentration is 0.01 mol L^-1,the average adsorption rates of Na⁺,K⁺,and Cs⁺on the surface of Cu²⁺-montmorillonite respectively 27 minutes are 4.48,8.68 and 11.74 mmol kg-1 min-1,and the equilibrium adsorption amounts are 129.17,341.11 and 911.47 mmol kg^-1.The average adsorption rate of Na⁺,K⁺,and Cs⁺on the surface of Zn²⁺-montmorillonite are 24 minutes 4.36,8.03 and 11.42 mmol kg-1 min-1,the equilibrium adsorption capacity are 115.15,238.06 and 784.79 mmol kg^-1,respectively.?3?The order of absolute effective charge coefficients of divalent alkaline earth metal ions is Ca²⁺?2.005?>Mg²⁺?1.425?.The order of absolute effective charge coefficient of monovalent alkali metal ions in Cu²⁺-montmorillonite system:Cs⁺?2.78?>K⁺?1.76?>Na⁺?1.14?,in Zn²⁺-montmorillonite system:Cs⁺?2.59?>K⁺?1.56?>Na⁺?1.11?.Whether it is a monovalent ion or a divalent ion,on the surface of montmorillonite saturated with different ions,due to the different adsorption energy of Cu²⁺,Zn²⁺and the surface,different Hofmeister effects are produced.The interaction between Cu²⁺and montmorillonite is stronger than Zn²⁺causes the apparent charge coefficient of alkali metal ions under the surface of the Cu²⁺saturated sample to be greater than that of the Zn²⁺saturated sample.?4?The effective charge coefficient reflects the difference between the adsorption energy of ions and the surface,which in turn leads to the adsorption selectivity of ions on the surface of montmorillonite,so that the kinetic parameters and electrochemical parameters in different systems show a significant Hofmeister effects.For covalent ions,the absolute charge coefficient of the cation is greater,it interaction with the surface is stronger.The greater the adsorption rate of ions,and the higher the adsorption amount when equilibrium is reached.The adsorption rate and equilibrium adsorption capacity of Ca²⁺and Mg²⁺on the surfaces of Cu²⁺-montmorillonite and Zn²⁺-montmorillonite are Ca²⁺>Mg²⁺,and the surface electrochemical parameters?₀??₀?E₀ of Ca²⁺-Cu²⁺and Ca²⁺-Zn²⁺systems are all greater than the corresponding values in Mg²⁺-Cu²⁺and Mg²⁺-Zn²⁺systems;the adsorption rate and equilibrium adsorption performance of Na+,K+,Cs+on the surface of Cu²⁺-montmorillonite and Zn²⁺-montmorillonite are Na⁺<K⁺<Cs⁺and the absolute value of surface potentials are expressed as?_Cs?0?<?_Na?0?<?_K?0?.?5?The Hofmeister sequence of the adsorption energy of Ca²⁺and Mg²⁺on the surface of Cu²⁺-montmorillonite and Zn²⁺-montmorillonite is Ca²⁺>Mg²⁺,Na⁺?K⁺?Cs⁺is Na⁺<K⁺<Cs⁺.Hofmeister energy in the interaction of Cs⁺-clay minerals plays an important role.The classical coulomb can play an important role in the interaction of K+/Na+-clay minerals.For example,at an electrolyte concentration of0.01 mol L^-1,the Hofmeister energy of Cs⁺,K⁺,and Na⁺on the surface of Cu²⁺-montmorillonite accounts for the proportions of the total interaction energy were 58.00%,42.19%and 8.82%,respectively;the proportion of Cs⁺,K⁺,Na⁺Hofmeister energy on the surface of Zn2+-montmorillonite to the total interaction energy was 55.53%,31.24%and 4.07%.Based on the above results,the following conclusions could be drawn:?1?The adsorption reaction of monovalent ions and divalent ions on the surface of Cu²⁺-montmorillonite or Zn²⁺-montmorillonite showed a strong Hofmeister effects and all followed the same Hofmeister sequence:Li⁺<Na⁺<K⁺<Cs⁺and Mg²⁺<Ca²⁺.?2?The Hofmeister effects are mainly due to the difference in the interaction energy between the ion and the surface caused by the non-classical polarization of the ion.?3?The type of ions adsorbed on the surface significantly affects the apparent charge coefficient of exchangeable ions on the surface of montmorillonite,and the Hofmeister effects also exists.